Meteorology 311

Meteorology 311
What is it?
RADAR
Transmits electromagnetic pulses toward target.
Tranmission rate is around 100 pulses per second.
Short silent period between pulses.
Pulse repetition frequency (PRF)
Measures the amount of beam that returns to the receiver.
RAdio Detection And Ranging
Called reflectivity.
Energy is scattered back to antenna by precipitation size particles.
Back-scattered energy.
Amount of energy return is primarily dependent on three factors:
1.
2.
3.
Size of target.
Distance from the target.
Wavelength of pulse.
Essentials
Rayleigh Scattering law
Back-scattered radiation ~ D6.
For collection of equal-size drops ~ 1/λ4.
Returned energy increases rapidly with
decreased wavelength.
The wavelength you use depends on what
you want to look at or resolve.
Recall what you learned in Mteor 301.
Weather radar: Typically 1 to 20cm.
3 cm for cloud detection, 10 cm for precipitation.
Reflectivity values
Reflectivity is measured in decibels (dBZ)
20 dBZ → Precipitation reaches the ground.
Important to realize for chasing.
Rainfall
20-45 dBZ : light to moderate.
40-50 dBZ : Pretty good rainfall.
60-70 dBZ : More than likely hail.
Snowfall
25-30 dBZ : Light to moderate snowfall.
30-35 dBZ : Moderate to heavy snowfall.
35-40 dBZ : Very heavy snowfall (rare) or rimed flakes.
Ice
Ice itself doesn’t have a very high reflectivity.
Why is hail so reflective?
Bright-banding
Operational Radar
A.k.a., the things you see on Gibson Ridge
software.
Current radars: WSR-88D’s (NEXRAD)
○ Weather Surveillance Radar made operational in 1988
and are of the doppler variety.
○ Previous radars were WSR-57’s and WSR-74’s.
○ 28 ft diameter dish that rotates at 360º at several tilts.
○ Dual polarization and phased-array are future radar
types soon to be implemented.
Course notes pictures.
Sites
159 Sites
Image provided by Rich Kinney, NWS, Des Moines
Images provided by Rich Kinney,
NWS, Des Moines
VCP
Volume coverage pattern
Modes: Convection, shallow precipitation, clear air.
Convection: Two digits starting with a 1.
Shallow precipitation: Two digits starting with a 2.
Clear air: Two digits starting with a 3.
○
Very sensitive, used when there is not precipitation detected in the coverage area.
Multiple PRF Dealiasing Algorithm Group: Three digits, starting with a 1
and followed by the two digit VCP with similar elevation angles.
VCP 31 and VCP 32
Clear air mode, 10 minute complete scan, 0.5 (base) - 4.5 degrees with
5 cuts (0.5, 1.5, 2.5, 3.5, 4.5).
http://www.weathertap.com/guides/graphics/radar/radar_vcp31.gif
VCP 21
Precipitation mode, 6 minute, 0.5 (base) - 19.5 degrees with 9 cuts.
http://www.weathertap.com/guides/graphics/radar/radar_vcp21.gif
VCP 11
Severe Weather (precip mode), 5 minute, 0.5 (base) - 19.5 with 14 cuts
http://www.weathertap.com/guides/graphics/radar/radar_vcp11.gif
Newer VCP’s
VCP 211, 212, 221
Same elevation angles as VCP 11, 12,
and 21.
Sachidananda-Zrnic Algorithm (SZ-2)
Reduces range ambiguity for Doppler data.
PRF
Pulse repetition frequency
Frequency at which pulses are emitted (pulses per second).
Essentially how long the ‘listening’ stage is (PRT = 1/PRF).
Determines the maximum unambiguous range and maximum unambiguous
velocity
Huh?
Huh, again?
Vmax = maximum velocity which a Doppler radar can determine unambiguously.
Purple haze: Radar cannot determine which pulse return signal may have
come from.
Rmax = maximum range which a transmitted pulse wave can travel and return to the
radar before the next pulse is emitted.
Also called “Range folding”.
Doppler Dilemma.
Products
Base reflectivity: Lowest elevation scan.
Composite reflectivity:
Mean wind field is “subtracted” from base velocity.
VIL
Spread in the velocity values.
Large spectrum width means a lot of turbulence.
Storm relative motion: Used to detect “mesoscale” circulations.
Provides basic wind field from the 0.5º elevation scan via Doppler effect.
Radar can only measure motions that have a component along a radial, or
essentially toward or away from the radar.
Spectrum width
Composed of the greatest reflectivity from any elevation angle seen from the radar.
Used to reveal the highest reflectivity in all echoes.
Base velocity
Basically used to tell what is out there.
Vertically integrated liquid water.
Amount of liquid water that the radar detects in a vertical column of the atmosphere
for an area of precipitation.
High values are associated with heavy rain or hail.
Echo Tops
Maximum height of precipitation echoes.
Base Reflectivity
Classic Supercell
N
Light Rain
Moderate/Heavy
Rain & Hail
Gust Front
Hook echo
Anvil Edge
Supercell Thunderstorm
(top view)
Nautical miles
0
5
10
Hook echo
WSR-88D Radar Image
Images provided by Rich Kinney, NWS, Des Moines
Composite Reflectivity
JetStream - An Online School for
Weather: RIDGE radar composite
reflectivity
Cool Stuff
Introduction to NEXRAD Anomalies &
Artifacts
TBSS
TBSS
National Weather Service Forecast
Office - Midland/Odessa Texas
Figure #16.
Bright Banding
Radar beam intersects freezing level and intersects
melting precipitation.
Image provided by Rich Kinney, NWS, Des Moines
Radar fine line
88D can detect subtle differences in humidity and temperature,
and the location of boundaries that may be the focus for storm
initiation or intensification
Image provided by Rich Kinney, NWS, Des Moines
AP
Anomalous propagation.
Dual Polarization
Two beams with different polarization directions:
one with horizontal polarization (current
polarization) and one with vertical polarization
New products useful for:
Precipitation type.
Rain and snow rate estimates.
Identification of aircraft icing conditions.
Reference: http://www.cimms.ou.edu/~kscharf/pol/
Page produced by Kevin Scharfenberg, who was also an invited speaker
at the 2008 Severe Storms and Doppler Radar Conference hosted by
the Central Iowa NWA.
ZDR – Differential Reflectivity
ZDR ~ 10 log (Ph / Pv ) [db]
Average shape of what you are measuring.
Typical values range from -2 db to 6 db.
Type discriminator
ZDR > 0?
ZDR < 0?
ZDR = 0?
Volume discriminator:
Same Z, high ZDR?
Same Z, low ZDR?
ρhv – Correlation Coefficient
Describes similarities in the back scatter
characteristics of the vertically and horizontally
polarized echos.
Rain: typically above 0.95.
Hail: 0.90 to 0.95.
Drizzle/very light rain: below 0.90
Ice pellets/graupel: typically above 0.95
Snow: typically above 0.95
Bright band: 0.90 to 0.95,
Good indicator of mixed precipitation regions
KDP – Specific differential phase
Takes advantage of difference in propagation
speeds in different media (water vs. air).
Takes range from radar into account to determine
where phase difference is occurring.
Proportional to φDP = φh – φv, called the differential
phase shift.
KDP>0, oblate hydrometeors.
KDP<0, vertically oriented hydrometeors.
KDP~0, spherical hydrometeors.
Good indicator of mixed precipitation regions and,
therefore, rainfall accumulation.